Typically, an automatic transmission includes a torque converter, a power train or gear shift mechanism that is connected to the torque converter, a hydraulic system for selectively driving operating elements of the power train, and a transmission control unit controlling the hydraulic system.
The hydraulic system of an automatic transmission typically includes various valves for driving operating elements of the power train using hydraulic oil exhausted from an oil pump. Such hydraulic system generally includes a regulator valve for regulating a pressure of hydraulic oil discharged from the oil pump to a predetermined level of pressure (i.e., line pressure). The regulator valve generally includes a valve housing with a plurality of ports, a valve spool disposed within the valve housing provided with a plurality of valve lands, and a coil spring elastically supporting the valve spool.
A position of the valve spool of the regulator is typically changed by an inflow of control pressure, thereby forming a specific line pressure. The line pressure that is formed by the regulator valve is then supplied to various valves.
Recently, a line pressure variable control technology has been developed that variably controls the line pressure. A variable force solenoid (VFS) has been introduced that supplies hydraulic oil to the regulator valve. That is, by controlling a duty ratio of the solenoid valve through an transmission control unit, the line pressure can be variably controlled. The object of the line pressure variable control is to improve efficiency of the transmission and fuel economy of the automobile by minimizing the line pressure in a state in which a specific gear is engaged. Therefore, in the line pressure variable control, it is preferable that a minimum line pressure exists at which there is no clutch slip.
Generally, the minimum line pressure is determined in consideration of a turbine input torque and a safety ratio. However, it can occur that a desired line pressure is not formed at a predetermined duty ratio of a solenoid valve due to a pressure deviation or aging of the hydraulic system. For example, though the solenoid valve is controlled at a specific duty ratio at which a desired minimum line pressure can be formed under normal conditions, the real line pressure can become less than the desired minimum line pressure. If the line pressure is too low, slips in friction elements of the automatic transmission or a damper clutch may occur.
[The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention, and should not be taken as an acknowledgement or any form of suggestions that this information forms the prior art that is already known to a person skilled in the art.]